Several methods are known in the art for controlling adaptive loops in communication receivers. For example, U.S. Pat. No. 6,920,189, whose disclosure is incorporated herein by reference, describes a carrier recovery (CR) loop in which a frequency-sensitive phase error signal representative of the frequency offset between the receiver and the carrier of the received digital input signal is generated. The phase error signal is then used to reduce the frequency error with respect to the received digital input signal to enable locking of the receiver to the carrier of the digital signal.
Other exemplary carrier recovery methods are described by Gagnon et al., in “A Simple and Fast Carrier Recovery Algorithm for High-Order QAM,” IEEE Communications Letters, (9:10), October 2005, pages 918-920, and by Kim and Choi in “Design of Carrier Recovery Algorithm for High-Order QAM with Large Frequency Acquisition Range,” IEEE International Conference on Communication (ICC), Helsinki, Finland, Jun. 11-14, 2001, volume 4, pages 1016-1020, which are incorporated herein by reference.
Receiver loops sometimes use decision-directed methods in which receiver decisions, such as demodulated symbols, are used in controlling the loop. For example, decision feedback carrier recovery loops for coherent demodulation of 16-QAM signal formats are described by Benany and Gagnon in “Comparison of Carrier Recovery Techniques in M-QAM Digital Communication Systems,” 13th Annual Canadian Conference on Electrical and Computer Engineering, Mar. 7-10, 2000, Halifax, Nova Scotia, pages 73-77, which is incorporated herein by reference.
In some methods and systems, weighting is applied to an error signal used for controlling the loop. For example, U.S. Pat. No. 6,826,238, whose disclosure is incorporated herein by reference, describes a quadrature amplitude modulation (QAM) receiver and carrier recovery method in which the frequency offset and phase jitter of a QAM signal are recovered using a weighted phase error inversely proportionate to the magnitude of the demodulated signal.